Electrostatic precipitator rapping system



p 30, 1958 H. J. WHITE ETAL 2,854,089

ELECTROSTATIC PRECIPITATOR RAPPING SYSTEM Filed Jan. 18, 1955 5 Sheets-Sheet 1 E g a m In g v Q In Q Q n Q E 3 Q 5 :s 2 Q m I 3 8 w INVENTOR H HARRY J. WHITE HERBERT J. HALL H B Y /JMTM ATTORNEY \llllll llllllll! I|| |||||||||||||\|||||I llllllllllll llll. v N

Sept. 30, 1958 H. J. WHITE ETAL ELECTROSTATIC PRECIPITATOR RAPPING SYSTEM Filed Jan. 18, 1955 5 Sheets-Sheet 2 INVENTOR HARRY J. W H ITE 'HEhBER J. HALL l 3Y HM ATTORNEY Sept. 30, 1958 JJWHITE ETAL 2,854,039

ELECTROSTATIC PRECIPITATOR RAPPING SYSTEM Filed Jan. 18, 1955 V 5 Sheets-Sheet 3 TIME TIME sscouos a 3 r 2 w D Q INVENTOR I HARRY J. WHITE 5 HERBERT J. HALL i: m g BY /-/M A ATTORNEY 5 Sheets-Sheet 4 W Wife...

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HERBERT J. HALL BY /MZ'M 3m 3m QE 8m 5m 8m N3 4/4/32 A 8N r H 3 wwm N 83 M 8 wmfi Sept. so, 1958 Filed Jan. 18, 1955 H. J. WHITE ETAL 2,854,089

ELECTROSTATIC PRECIPITATOR RAPPING SYSTEM 5 Sheets-Sheet 5 HARRY J. WHITE HERERT J. HALL BY 4M1 4M ATTORNEY rapping puffs or stack clouding.

United States Patent ELECTROSTATIC PRECIPITATOR RAPPING SYSTEM Harry J. White, Basking Ridge, and Herbert J. Hall, Princeton, N. J., assignors to Research Corporation, New Yorlr, N. Y., a corporation of New York Application January 18, 1955, Serial No. 482,636

6 Claims. (Cl. 183-7) This invention relates to an electrostatic precipitator rapping system, and more particularly to a novel electrode rapping device and novel electrical circuit means for energizing same. The present application is in part a continuation of co-pending application, Serial No. 164,468, of the same inventors, filed May 26, 1950, for Rapping Device and System, now abandoned.

Rapping methods used in the past have been intermittent, that is, dust or fly ash which gradually accumulated on the electrodes was shaken loose at periodic intervals. With this intermittent rapping and the plunger efiect of large quantities of dust dropping into the hoppers, there was considerable dust re-entrainment which resulted in It is an object of the present invention to provide a solution to the above difficulty, and to provide a system for automatically and continuously cleaning precipitator electrodes, to eliminate rapping puffs, maintain optimum precipitator electrical conditions, increase daily collection, improve community relations, and reduce operating costs.

A further object is to provide a rapper and rapping system that are simple and inexpensive to manufacture and to maintain in operation and that are very reliable in use.

. Typically, the rapping device of the invention includes a sleeve, a plunger of magnetic material in the sleeve and slidable therein, stop members limiting longitudinal movement of the plunger in the sleeve, a magnetizing coil wrapped about the sleeve, and means for attaching the sleeve to an element to be rapped. The plunger may have a longitudinal opening, preferably a slot, extending therethrough for purposes to be explained hereinafter. Preferably also the sleeve has end closure members of non-magnetic material that serve as stop members for limiting movement of the plunger. As will appear more fully as the description proceeds, the rappers of the invention may be used to advantage on either the grounded or on the high tension electrodes of an electrical precipitator.

The rapping system of the invention involves an energizing apparatus so related to the rapping device that maximum efficiency of operation is obtained. The energizing apparatus includes the magnetizing coil element of the rapping device and provides energizing current pulses of controlled form, duration and interval. According to the invention, electric low tension power, usually from a commercial source, flows through a remote control unit to a pulse generator. By means of a transformer and full-wave rectifier in the pulse generator, the alternating voltage is increased and then changed to direct-voltage. The power supplys directcurrent output, which is smoothly regulated by the control unit, passes through a resistor and charges an energy storage condenser to the desired voltage. The condenser sends out periodic discharges, which are timed and controlled by a thyratron and a distributor switch, to the various rapper unit coils. The pulsing discharges excite these coils in regularly timed succession. The steel plungers of the individual rappers are thus attractedthrough the excited coils to .deliver the rapping impact, at'regu larly timed intervals, throughthe rapper bars to'the collecting plates of the precipitator. v I

Thepresent invention converts precipitation from'a batch process to a continuous, uniform proc'ess.' The collecting plates are ,grouped in banks and each bank is rapped in sequence with a fast rapping cycle of a minute or less. The collected, dust-aggregates, which are "looserred by frequency-and-ihtensitykcontrolled rapping, be come dislodgedin small isolated patches. These tend to fall close to the plates into the hoppers below. result, only small, easilyreprecipitated quantities arejs'wept into the gas stream. This continuous, sequential rapping of closely c'ontrolled intensity promotes stable, voptimum electrical conditions in the precipitation zone; The amount of dust falling at'any given time is smallenough to be easily accepted and retained bythe hoppers. Puffs are eliminated and uniform precipitator'performance is achieved to meet present-day demands for constantly clear stacks. p

The specific nature of the invention, as well as other objects, and advantages thereof, will clearly appear from a description of a preferred embodiment as shown in the accompanying drawings in which:

Fig. 1 is a schematic'drawing showing the arrangement of the overall system and the manner in which a rapper is attached to the electrodes of a precipitator;

Fig. 2 is a longitudinal cross-sectional viewof a rapper. unit;

Fig. 3 is a schematic circuit diagram of the control and energizing system for the rappers of a precipitator;

Fig. 4 is a'diagrammatic view of a typical electrical circuit for energizing the rapping devices of the invention,

showing the principle of operation;

Fig. 5 is a graph showing the form of a typical electrical current pulse as supplied by the circuit of Fig.4;

Fig. 6-is a'diagrarnmatic view of another electrical energizing circuit;

Fig. 7 is a graph of a pulse as supplied by the circuit of Fig. 6; I

Fig. 8 is a: longitudinal sectional view of a'rap'ping' 4 Referring to Fig. 1 the control unit 2 is energized from any suitable low-tension power source 4, usually a commercial source of from -440 volts, Control unit 2 contains-circuit elements shown in more detail in Fig. 3, and provides on its front panel 3, a manual control knob 6 for a suitable voltage control device, e. g.,

.an. autotransformer, and panel voltmeter 8 for showing the voltage to which the autotransformer-is set, and a suitable pilot or signal light 9 for indicating proper operation-of the system, as well as control switches 10 and 12. Controlled poweris 'thus. supplied, as schematically indicated by lead 14, to pulse generator 16, which contains, arectifier 18, charging storage condenser 20, and

resistor 22 through which the condenser is charged. The condenser 20, sends out periodic discharges which are timed and controlled by thyratron 24 and distributor switch 26 cyclically driven by motor 27, as will be explained in detail below, to the various rapper unit coils. Atypical rapper 28 is shown in the process of being energized through distributor switch 26. Rapper. 28

is, integrally connected to a rapping bar- 30, which passes 'throughbtufling box 32' attached to the precipitator external wall 34 to prevent gas leakage through the wall. Rapping bar 30 is welded, as shown at 36 and 38, to the edges of parallel precipitator collector plates 40 and 42, which constitute typical grounded collector electrodes of a precipitator. Between the collector electrodes 40 and 42, etc., are spaced discharge wires 41, 43, etc., constituting the discharge electrodes of the precipitator. As the outermost of these is set in a distance from the edges of plates 40, 42, etc., there is adequate electrical clearance between the high tension discharge electrodes 41, 43, etc., and the nearest portion of the grounded rapper bar 30.

Fig. 2 shows the detailed construction of a typical rapper unit 28. The exterior casing 46 is provided with a flange 48 drilled to accommodate a number (e. g., four) of bolts 50 for fastening to flange 53 of block 52, by means of nuts 54, preferably of shakeproof construction, bearing against a washer 56 to compress an isolating pad 57 of suitable resilient material, an isolating bushing 58, and gaskets 59, so as to make an impervious gas-andmoisture tight seal. Conductors 62 and 63, for energizing the magnetic coil 64 of the rapper unit, pass through closely fitting holes in compressible sealing disc 66 so that when connector 67 is screwed in tight against shoulder 68 of fitting 69, the disc is compressed to make a moisture-tight seal both around the wires 62 and 63 and also with the fitting 69. Thus the construction is completely sealed against atmospheric dirt, moisture or corrosion. Plunger 71 moves freely in tube 72 which is preferably graphite-impregnated for lubrication. Spring 73 returns the plunger to the position shown, while coil 64, when energized with electric current, draws the plunger to the right as seen in Fig. 2, until its head, which is hardened and chamfered to prevent burring, strikes Stellite insert 74 in block 52 to transmit a rapping impulse to rapper bar 30. A slot 75 is provided axially of the plunger to reduce eddy currents and provide an air passage for returning movement of the plunger. The Stellite insert or button 74 is non-magnetic to minimize any tendency of the plunger to stick to block 52 and hinder its return under the action of spring 73. Coil 64 is tightly fitted between an insulating tube 77 (which may be made of Bakelite) which surrounds tube 72, and the exterior casing 46. Rapper bar 30 is rigidly fastened, as by welding, at 78, to block 52 to form a unitary construction for efficiently transmitting the rapping impulse to the precipitator.

The function of gaskets 59 is important, as they not only serve as a seal, but also provide cushioning between the casing 46 and the block 52, which intensifies the rapping blow for a given current. The magnetic flux path, however, is returned from the block to the casing through the close-fitting slide contact between the two at 79.

A cushioning gasket 59' may be provided to provide cushioning between the casing 46 and the plunger 71.

Figure 3 shows a practical circuit arrangement of Fig. 1. The circuits of control unit 2 are indicated by the broken line, and comprise terminals 82 for the incoming power line 4. An intermediate transformer 84 is provided with suitable switching schematically indicated at 86 for transforming the supply voltage to 115 volts, 6O cycle, for which the pulsing circuit is designed. If 115 volt power is available, transformer 84 may, of course, be omitted. Autotransformer 86' is controlled by knob 6 on panel 3 (Fig. l) to set the voltage at the correct operating level, and voltmeter 8 is provided for checking on this voltage. Leads 87, 88, 89 and 91, are provided to connect the control unit with the rest of the precipitator rapping system. Power transformer 92 is energized from the output of autotransformer 86 over leads 88 and 89, and is, therefore, under control of the autotransformer. Supply transformer 93 is energized directly from the line over lead 87 and its primary winding is returned to ground at 88. Transformer 93 has a filament winding 94 for the filaments of full-wave high-voltage rectifier 96, 97, which may be kenetrons or any other suitable high-voltage rectifiers; a second filament winding 98 for thyratron 24 is provided, and a charging Winding 101 for the thyratron grid control circuit which will be described below. In the control panel a line switch 99 is provided for the entire circuit, and a rapper switch 102 for the direct-current charging circuit of the pulse generator.

The output of the full-wave rectifier circuit is suppliedthrough charging resistors 22, which in practice may be comprised of four smaller resistors of a value suitable to the current which they must carry in charging the heavy-duty capacitor 20, which is typically a 120 microfarad 3000 volt direct-current condenser. Capacitor 20 is shunted by a one mcgohm, 4 watt resistor 21. A shorting switch 22 is connected across resistors 22 and capacitor 20 and the switch may be door operated for safety purposes. 115 volt alternating-current timer motor 27 is connected across lines 87 and 89 and is therefore under control of switch 99. Transformer secondary 101, of approximately 60 volts output, is connected to the grid of thyratron 24 through rectifier 103, and resistors 104, 106 and 107, to provide a negative bias for the thyratron. Smoothing condenser is provided to maintain the bias substantially constant. A parallel circuit from transformer secondary 101 is provided through rectifier 108, oppositely oriented to rectifier 103, and through resistor 109 to grid condenser 111, which is alternately connected by timer motor 27 and switch 110 through any suitable driving mechanism (not shown) to contacts 112 and 113. Thus the grid condenser 111 is alternately charged positively through rectifier 108 and connected to the thyratron grid to cause discharge of condenser 20 through the thyratron, to ground and through the rapper coil which has at that time been connected by distributor switch 26 to ground. As switch 110 is alternated between contacts 112 and 113 during the time between the closing of two successive rapper contacts, it will be clear that the full charge-discharge cycle of the condenser 20 will take place each time the circuit is closed to each successive rapper. When the condenser 20 discharges below the firing plate to cathode potential of thyratron 24, conduction through the thyratron will cease, and the system will be ready for the next cycle.

Pilot lamp 116 is connected across resistor 117, which is in series with the resistor 118 and condenser 20, so that the lamp will flash when the condenser discharges, thus giving a further indication at the control unit 2 of the operative condition of the system. The individual rappers 28a to 28a are connected successively to condenser 20 by switch 26, here shown schematically as a plurality of separate switches successively closed by timing motor 27 through any suitable known mechanism.

Fig. 4 shows a simplified form of a circuit arrangement which is functionally similar to that of Fig. 3, and which shows in more elementary form the principle of operation of the rapper circuit. A direct-current power supply is indicated by the reference numeral 138, and corresponds to the power transformer 92 and rectifier tubes 96, 97, of Fig. 3. The magnetizing coils of the rapping units are designated 128a to 128s.

There are two parallel circuits across the terminals of the direct-current power supply. The current limiting or charging resistor 122 is common to both parallel circuits. One of the circuits includes a pulse condenser and, selectively, the solenoid coils 12811 to 128e, each of which is separately included in the circuit by a distributing switch 126 making contact with any one of a plurality of switching terminals 127. The other of the parallel circuits includes an on-off switching device in the form of a thyratron 124 providing a shunt path for discharging the pulse condenser 120 through the solenoid windings in turn.

The thyratron may be controlled by a conventional trigger circuit. As shown, cut-off bias is maintained on the thyratron grid by a battery 203 (corresponding to the negative bias supply including rectifier 103 of Fig. 3), the negative terminal of which is connected to the grid through a current limiting and isolation resistor 206' and the positive terminal of which is connected to the thyratron cathode. The firing battery 208, of larger voltage than the biasing battery 206, has its positive terminal connected to the thyratron grid through a current limiting resistor 209 and its negative terminal connected to the cathode through a switch 210. When the switch 210 is momentarily closed, a positive charge develops on the grid of the thyratron and the tube becomes conductive; the tube returns to the non-conductive state due to the reversal of anode voltage polarity which occurs as a result of the condenser discharge through the solenoid coil. The distributing switch 126 and the firing switch 210 could be manually operated, but are preferably power driven and synchronized by appropriate timing devices.

In operation, starting with the apparatus in the condition depicted in Fig. 4, the pulse condenser 120 is allowed to reach substantially full charge; the time required will depend on the time constant of the series system including resistor 122, condenser 120 and coil 128a. The switch 210 is then closed and the thyratron fires allowing the pulse condenser to force a pulse of current through the coil 128:: to actuate the rapper plunger associated therewith. During firing, the resistor 122 prevents substantial short circuiting of the directcurrent power source through the thyratron. The distributor 41 is shifted to include the coil 3128b in the circuit, and the firing switch is opened to permit the pulse condenser to be charged, and the firing switch is then closed to energize the coil 12% with a current pulse. This sequence of operations is continued until all the rapping units have been operated and the cycle is then repeated as desired.

The construction of switch and contacts 126127 is such that at all times a closed circuit is maintained from capacitor 120 via the solenoid coils to the thyratron cathode. A similar construction is preferred in the distributor switch 26 of Fig. 3 of the drawings.

It will be apparent that the above circuit is a simplified version of the circuit shown in Fig. 3, in which, instead of two batteries, rectifiers 103 and 108 are employed, while the actual switching circuit is slightly difierent in that rectifier 108, instead of directly firing the thyratron, is used to charge condenser 111 for this purpose.

If maximum rapping efiiciency and, in deed, if an operative system is to be obtained, the physical, magnetic and electrical parameters of the rapping system must be properly interrelated. The duration and form of the current pulse should be suitably related to the transit time of the plunger, which in turn is determined by the physical parameters of the rapper unit and the rapping energy desired to be obtained at each stroke of the plunger.

Referring to Fig. 5, the solid curved line is the currenttime plot of a typical energizing pulse as supplied by the system of Fig. 4. The current must reach a value i before the magnetic force on the plunger is suflicient to overcome friction. At is the time required for the current to reach the minimum required to start the plunger from rest. The plunger is accelerated during the time Ai during which the currrent is greater than 1' In general, it is desirable that the current shall have decayed to a value substantially lower than 1' at the time that the plunger strikes the stop member, this for the reason that magnetic forces tend to snub the rebound of the plunger and to cause a substantial loss in effectiveness unless the magnetic force at and just after impact is low.

This requires the current at impact to be low; a value such as i in Fig. 5 being typical. The electrical damping of the resonant circuit formed by condenser 120 and inductance 128 must be low enough. to insure oscillatory discharge which is necessary to cause thyratron 124 to cut off.

It is thus seen that the transit time of the plunger is represented by At and that the acceleration time is represented by M The plunger transit time is longer than the acceleration time; the latter must not be too short in order thatthe magnetic flux shall have sufiicient time to penetrate the plunger. The pulse time, T, is arbitrarily found by extrapolating the downward portion of the curve from the time of plunger impact to the base line.

The pulse amplitude i is controlled by the voltage applied to the pulse condenser 40 and the pulse shape and duration are dependent primarily upon the capacitance of the pulse condenser and the inductance and resistance of the magnetizing coil.

From the foregoing considerations, it will be seen that the acceleration time is less than the plunger transit time which, in turn, is less than the pulse time. Typically, the acceleration time is about one-half the pulse time. It will be evident that additional inductance or capacitance may be inserted in series with the pulse condenser and solenoid coil to vary the pulse time.

By way of example and without limitation, the following numerical values of the components of a typical rapper system are given:

Plunger:

Length 7.5 inches. Diameter s 1.43 inches. Weight 3.03 pounds. Material Solid CRS steel, slotted, and annealed after machining. Length of plunger in coil at start of stroke 2.0 inches. Length of stroke 4.0 inches. Magnet coil:

Winding length 5.5 inches. Form diameter 1.5 inches. Radial winding depth 0.45 inch. Wire size #29 H. F. Number of turns 5810. Coil resistance 234 ohms. Initial inductance with plunger 0.55 henry. Inductance at impact 0.84 henry.

Circuit conditions (using additional inductance in series with magnet coil):

Total initial inductance 1.51 henries.

Total resistance 273 ohms. Storage capacity 128 mfd. Condenser voltage 2340 volts. /2 CV 350 joules. Kinetic energy of plunger at impact 6.82 joules. Momentum of plunger at impact 0.97 lb. sec. Peak current in magnet coil 6.0 amperes.

The energizing circuit shown in Fig. 6 is similar to that shown in Fig. 4. In Fig. 6, parts corresponding to similar parts in the system of Fig. 4 bear corresponding primed reference numerals. It will be seen that the system of Fig. 6 includes a small parallel resonant circuit having a coil 250 and a condenser 251, the parallel resonant circuit being connected in series with the pulse condenser and the solenoid 128a, etc.

The effect of this additional parallel resonant circuit on the pulse form is illustrated in Fig. 7. It will be seen that the current pulse has relatively steep rise and decay portions and a relatively fiat peak. The flat topped current pulse may be utilized where it is desired to maintain the current at a fairly uniform high value during the plunger acceleration time and to have the current de- 7 cay quickly to a relatively low value before plunger impact.

Where the rapping device of the invention is used in a horizontal position, gravity cannot be employed to return the plunger to starting position and some other force must be substituted. in the rapper shown in Fig. 8, a solenoid coil returns the plunger to starting position.

Referring to Fig. 8, the rapper mounting frame is shown at 252. A cylindrical steel shell 253 is received in a circular opening 254 in the frame and is welded therein. An end closure disc 255 of steel is welded in the left-hand end of the shell and a non-magnetic stop disc 256 of stainless steel is welded to the central portion of the end closure member 255. The plunger 257 is carried in a brass cylinder 258 that is supported at the left-hand end by the stop member 256. The right-hand end of the cylinder is supported on a stainless steel disc 259 screwed to a cap that, in turn, is screwed over the threaded end of the shell 253 and retained in place by a lock washer 261 and lock nut 262.

A Bakelite coil form 263 is fitted over the plunger guide cylinder and carries the pulse coil 264 and plunger return coil 265. Leads from the coils are brought out through openings 266 and 267 in the shell. A stainless steel washer 268 fits over the right-hand end of the coil form and together with the disc 259 eliminates unduly large magnetic forces between the right-hand end of the plunger and the stop member 260 at the right-hand end of the shell when the plunger is in rest position as shown.

The plunger return coil may be continuously energized with direct current whereby the plunger is normally retained at rest in the position shown in Fig. 8. When a controlled pulse of current is passed through the pulse coil 264, the plunger is accelerated to the left and strikes the stop member 256, thus creating the desired rapping action. After decay of the pulse, the constant magnetic field of the return coil 265 returns the plunger to initial position.

It is contemplated that, alternatively, the return coil 265 may be intermittently energized with controlled pulses of current which act in timed relationship with the pulses supplied to the coil 264 to cause the plunger to oscillate continuously from one end of the cylinder to the other. Enhanced effects are obtained if the alternate pulse frequency is close to the natural frequency of rebound of the plunger from the ends of the cylinder.

Fig. 9 shows the energizing system in the case where the high tension discharge electrode is rapped instead of (or in addition to) the grounded collector electrode. In this case it is necessary to interpose a highly insulated transformer winding between the condenser charging system and the rapper, since the rapper must be mounted on the supporting member for the high voltage discharge electrodes in order to effectively transmit rapping impulses to the electrodes; therefore, the rapper and its leads possess the high charge of the high tension electrodes which may be in the order of 60,000 volts. It is, therefore, not feasible to connect the coil of the rapper directly to the source of controlled current pulses. Instead, an insulating pulse transformer 286, the secondary circuit of which is insulated from the primary circuit by insulation capable of withstanding 80,000 volts or more, is inserted between the pulse source and the rapper.

In Fig. 9, the energizing system circuit including the pulse transformer is shown diagrammatically. The pulse transformer secondary winding 287 is connected by the wires 285 to the solenoid coil 328 of the rapper. One end 289 of the primary winding 290 of the pulse transformer is grounded; the other end 291 of the primary winding is connected through a shielded cable 292 to one terminal of the pulse condenser 293. The other terminal of the pulse condenser 340 is connected to the anode of a thyratron 324 and to the positive terminal of the directcurrent power supply through a charging resistor 322.

'8 The negative terminal of the power supply and the cathode of the thyratron are grounded as shown.

In operation, a triggering potential is supplied to the grid of the thyratron and the pulse condenser is discharged through the primary winding of the pulse transformer. The pulse induced in the secondary winding of the transformer is applied to the magnetizing coil of the rapper to actuate the plunger of the latter in the manner described hereinbefore.

Fig. 10 shows another form of rapper which is for vertical mounting only, and may be attached to each collecting electrode plate by means of a vibration-transmitting rod 315, the lower end of which (not shown) is welded or otherwise rigidly attached to the plate electrode. The upper threaded end 316 of the rod is received in a correspondingly threaded hole 317 in the circular base member 318 of the rapper. A lock nut 319 forces a lock washer 320 against the bottom of the base member to secure the rod to the base member.

A vertically extending cylindrical casing 321 is fitted to the circular base member 318 and is welded thereto as shown at 322. The top of the base member has a circular recess 323 formed therein and a disc 324 of nonmagnetic material or metal such as stainless steel is seated in the recess.

An inner sleeve or cylinder 325, formed of a nonmagnetic material, such as synthetic resinous material, for example, Bakelite, or brass, rests upon the disc 324 and extends upwardly nearly to the top of the rapper. A reinforcing sleeve 26, which may also be formed of Bakelite or the like, surrounds the lower half of the inner cylinder and may be cemented to the latter.

Around the upper half of the inner tube 325 there is wound a magnetizing or solenoid coil 327. The coil is retained in place by Bakelite or similar rings 328 and 329 cemented to the tube 325. Lead wire 330 from the coil ends extend downwardly through holes drilled in the lower rings 328 and the wires are lashed to the tube 326 by a glass fiber cord 331. The lead wires 330 are carried to the source of energizing current through the electrical conduit 332.

A plunger, piston or armature 333, formed of magnetic material such as iron or steel, is received in the cylinder 325. The outside diameter of the plunger is slightly smaller than the inside diameter of the cylinder so that the plunger can rise and fall freely in the cylinder.

The ends of the plunger may be tapered as shown toprevent binding and facilitate passage of the plunger through the cylinder. The plunger is provided with a radial slot 334 extending from end to end of the plunger, which slot may be milled to depth approaching the principal axis of the plunger. The slot reduces eddy current losses in the plunger and provide a by-pass path for air as the plunger moves through the cylinder.

A disc 335 of stainless steel or other non-magnetic material closes the top of the cylinder and is retained in place by a circular cap piece 336 that is welded as shown at 337 to the casing 321.

The base member 318, casing 321 and cap piece 336 preferably are fabricated of ordinary low-carbon steel.

From the foregoing description, it will be evident that when a pulse of current is passed through the coil 327 the plunger 333 will rise and strike the end closure or stop member 335. The vibrations resulting from the impact will be transmitted, largely through the cap piece 336, the casing 21, the base member 318 and the rod 315, to the electrode plate or other member to be vibrated. Additional vibrations are created when the plunger rebounds and strikes the base member 318.

Instead of employing an isolating pulse transformer, as in Fig. 9, the rapper unit of this invention may be used for rapping the high tension electrodes of electrical precipitators if the solenoid coils of the rappers are adequately insulated from the highly charged portions of the high tension electrode system. If this precaution is 9 taken, the wires from the magnetizing coils may be connected directly to the pulse condenser without the necessity for interposing an isolating pulse transformer. Insulated constructions for this purpose are illustrated in Figs. 11 to 13.

In the rapper of Fig. 11, the plunger is lifted against the upper stop member by the magnetic impulse from the magnetizing coil and allowed to rebound with substantial force against the high tension electrode support bar. The high tension electrode support bar 340 carries a nonmagnetic stop member 342 in the form of a disc. A gasket ring 344 lying on the stop member supports a cylinder 346 having a base flange 348 secured to the electrode support bar 340 by screws 350. The cylinder 346 is formed of ceramic or other insulating material and has a cap 352 threadedly engaging the top thereof. Depending into the bore of the cylinder there is a small helical spring 354 aflixed to the bottom of the cap. A magnetic plunger 356 is loosely fitted into the bore of the cylinder.

The cylinder has a flange 358 supporting a solenoid coil 360 protected by a casing 362. Lead wires 364 conduct pulses of energizing current to the coil. Because the coil 360 is completely insulated from the highly charged bar 340, the lead wires 364 may be connected directly to the current pulse source without involving danger of shock from the high tension member.

When a pulse of current is passed through the coil 360, the plunger 356 is drawn up. It strikes the spring 354, lightly if at all, and rebounds against the stop member 342 thus setting up vibrations in the electrode support bar 340.

In the form of the invention shown in Fig. 12, the rapper unit is mounted beneath the high tension electrode support bar 366 on a grounded base member 368. The insulating cylinder 370 has abase flange 372 that is attached to the base member 369 by screws 374. A gasket 376 and impact disc 378 are pressed between the cylinder flange and the base member.

The top of the cylinder 370 stops short of the bottom of the electrode support bar 368 and a sylphon bellows 380 is fitted between the top of the cylinder and the bottom of the bar. An impact or anvil member 382 is welded to the bottom of the bar.

In common with other forms of the invention, a magnetic piston 384 is fitted in the bore of the cylinder and a magnetizing coil 386 is wrapped about the outside of the cylinder.

Operation of the unit of Fig. 12 is believed to be apparent. A pulse of current is passed through the coil 386. The plunger is urged upwardly and into striking engagement with the anvil 382 to vibrate the electrode support bar 366. The plunger then drops back to rest position preparatory to the initiatoin of another cycle of operation.

The coil 386 and the terminal wires 388, being insulated from the high tension electrode system, may be safely connected directly to the current pulse source.

Referring to Fig. 13, the floor 390 of an insulator compartment of an electrical precipitator carries a main high tension electrode support 392 on insulating columns 394 and 396. A metallic pipe support and lower plunger guide 398 is welded in a hole in the electrode support 392 and depends through a wide hole 400 in the floor of the insulator compartment into the subjacent precipitating chamber 402. An insulating bushing 404 surrounds the pipe 398 and is supported by the floor 390. Elbow hangers 406 and 408 are welded to the lower end of the pipe support and a high tension electrode support bar 410 is attached to the lower ends of the elbow hangers. From the support bar 410, the usual high tension electrode wires 412 are suspended.

Beneath the lower end of the pipe support 398 an anvil or butter plate 414 is welded to the top of the electrode support bar. A sylphon bellows 416 is con- I116 nected between the bottom of the pipe and the'anvil member.

In the top 418 of the insulator compartment a large hole is cut concentrically with the axis of the pipe 398. An inverted cup-shaped upper guide tube support 420 is mounted over the hole. The cup-shaped support carries an upper guide tube 422 of insulating material that projects through a hole in the support and extends coaxially of the tube 398, forming in eifect an extension of the latter. The upper guide tube has a removable top cap 424 and is connected at the bottom by a sylphon bellows 426 to the top of the lower guide tube 398.

Wrapped around the upper guide tube is a solenoid coil 428. A metal shield and protective cover 430 is placed over the top of the upper guide tube and is fastened to the cup-shaped support 420 by screws.

A. rapping plunger generally designated 432 is received within the cylinder comprised of the lower and upper guide tubes 398 and 426 respectively. The plunger has a lower metallic striker end 434, an intermediate insulating portion 436 formed of synthetic resin or the like, and an upper portion 438 formed of a magnetic metal. The top of the upper portion of the plunger projects slightly within the solenoid coil when the piston is at rest, and the upper magnetic portion extends substantially below the bottom of the coil but terminates above the bottom of the upper insulating guide tube 422. The intermediate insulating portion of the plunger extends downwardly into the lower guide tube 398 for a distance below the bottom of the upper guide tube that is somewhat greater than the stroke of the plunger. Thus a substantial portion of the intermediate insulatingv section of the plunger will lie between the bottom of the upper tube 422 and the bellows 426 for all positions of the plunger in the cylinder, thus preventing grounding of the highly charged elements of the electrical precipitator through the plunger.

In operation, a controlled pulse of current is passed through the coil 428. The magnetic field produced acts upon the magnetic portion 438 of the plunger to draw the plunger upwardly in the cylinder. The plunger then falls back to strike the anvil member 414 and to set up vibrations in the electrode support bar 410 and to shake accumulated deposits from the electrode wires 412.

It will be apparent that the embodiments shown are only exemplary and that various modifications can be made in construction and arrangement within the scope of our invention as defined in the appended claims. For example the spring return type rapper shown, for example, in Fig. 2 of the drawings may be used in the vertical position to rap high tension electrodes or return springs may be employed in the devices shown and Figs. 11 through 13.

We claim:

1. In combination with an electrostatic precipitator having electrodes which accumulate precipitated material, an electrode rapping system including a rapping unit comprising a sleeve, a plunger of magnetic material in said sleeve and slidable therein, stop members limiting longitudinal movement of said plunger in said sleeve, a magnetizing coil wrapped about said sleeve, a pulse condenser, a thyratron, circuit means connecting said pulse condenser to discharge through said coil and said thyratron when the latter is fired, trigger circuit means for firing said thyratron, a direct current power supply connected through a high resistance to charge the circuit including said pulse condenser, coil and thyratron having a relatively low effective resistance whereby a single unidirectional pulse of discharge of said condenser occurs, means rigidly connected between one of said stop members and electrodes of said precipitator for transmitting impulses to said electrodes, for dislodging accumulated material therefrom.

2. The invention according to claim 1, said trigger circuit means for firing said thyratron comprising means for negatively biasing the grid of said thyratron to prevent firing thereof, and means for periodically superimposing a positive bias on the negatively biased grid to fire the thyratron.

3. The invention according to claim 2, said last means comprising a source of positive potential, means for periodically connecting a condenser to said source to charge the condenser, and means for periodically conecting said charged condenser to the thyratron grid to impose a positive bias on said grid.

4. The invention according to claim 3, and a plurality of further rapper units each connected to respective further electrodes of said precipitator for transmitting rapping impulses thereto, a commutating switch for successively connecting said pulse condenser to discharge a single unidirectional pulse through the coil of each rapper unit in succession and through said thyratron when the latter is fired, and means for energizing said trigger circuit in synchronism with said commutating switch.

5. in combination with an electrostatic precipitator having electrodes which accumulate precipitated mate rial, an electrode rapping system including a rapping unit comprising a sleeve, a plunger of magnetic material in said sleeve and slidable therein, stop members limiting longitudinal movement of said plunger in said sleeve, :1 magnetizing coil disposed about said sleeve, a pulse condenser, a normally non-conducting electron discharge device, circuit means connecting said pulse condenser to discharge through said coil and said electron discharge device when the latter is rendered conductive, trigger circuit means for rendering said electron discharge device conductive, a direct current power supply connected through a resistance to charge the circuit including said pulse condenser, coil and electron discharge device having a relatively low effective resistance whereby a single unidirectional pulse of discharge of said condenser occurs, and means rigidly connected between one of said stop members and electrodes of said precipitator for transmitting impulses to said electrodes for dislodging accumulated material therefrom.

6. The invention according to claim 5 and further means including a resonant circuit means comprising a parallel arrangement of an inductance and a capacitance connected in series between said coil and said pulse condenser.

References Cited in the file of this patent UNITED STATES PATENTS 773,125 Christmas Oct. 26, 1904 852,926 Carver et al. May 7, 1907 1,760,461 Weyandt May 27, 1930 1,762,811 Charlton June 10, 1930 1,790,147 Heinrich Jan. 27, 1931 1,912,053 Winterrnute May 30, 1933 2,508,134 Anderson May 16, 1950 2,508,973 Smith May 23, 1950 2,525,872 Dawson Oct. 17, 1950 2,550,809 Hedberg May 1, 1951 2,699,224 Schmitz Jan. 11, 1955 FOREIGN PATENTS 216,789 Great Britain June 5, 1924 

